Printed circuit (PC) boards, like everything else in electronics, are getting smaller and denser. A compact packaging technology is needed when mounting area is limited dictating that circuit elements be closely spaced. A module consisting of a plurality of semiconductor devices is used to densely dispose semiconductor devices and to obtain a small-sized electronic apparatus. PC boards have typically been designed so that semiconductor devices in the form of packaged semiconductor dice are mounted thereon to make a final circuit. Mounting can be accomplished by a surface mount technique or by socketing. Interconnections among devices arrayed on PC boards are usually made by conductive traces formed by photolithography and etching processes.
Additionally, semiconductor dice are traditionally probed individually before assembly, while critical units are burned in under accelerated aging conditions to minimize the risk of subsequent system failure. Burn-in is performed to screen out weak devices, and packaged devices rather than bare chips are normally burned-in. In a multiple chip module, the burn-in process should be performed at the packaged module level. The drawback in module level burn-in is that a percentage of die in the module will fail, and replacement with another good die has to be performed by a proper removing procedure. The replacement procedure may not always be cost effective or easy to effect.
One circuit board mounted array of semiconductor devices of particular interest is the single in-line memory module (SIMM) or a memory card. SIMM boards or memory cards are circuit arrays which are typically byte multiples of memory chips arranged on a daughter PC board or comparable mounting arrangement. The SIMM board or memory card is connected to a circuit control board by an edge connector. One edge of a SIMM module is a card edge connector, which plugs into a socket on the computer which is directly connected to the computer busses required for powering and addressing the memory on the SIMM. The semiconductor devices on SIMM boards are typically packaged in small outline J-leaded packages (SOJs) or thin small outline packages (TSOPs) before they are mounted on the daughter PC board.
Single in-line packages (SIPs) are similar in design to SIMMs, except that instead of having a card edge-type connector, SIPs have pins which are either socket mounted or solder mounted to a mother board or bus. These modules have been constructed by first packaging individual semiconductor dice into packages, and then soldering the packaged dice onto a daughter PC board. These packages are attached to the daughter board by surface mount techniques or into through-holes.
The disadvantage to both SIMMs and SIPS is that the semiconductor dice have to be packaged individually before they are mounted on a daughter board. Furthermore, each device is tested and burned-in on a discrete unit level instead of a module level.
Several methods exist for fabricating a semiconductor module. One such method uses a cofired ceramic substrate, onto which bare semiconductor dice are directly attached to the ceramic mounting surface and are wire bonded to conductive areas on the mounting surface, or are inverted and connected directly to metallized areas on the ceramic mounting surface by, for example, a solder-bump technique. However, direct chip attach has a limitation of no burn-in capability before module assembly and is difficult to repair after board mounting.
Another method of fabricating a semiconductor module involves tape automated bonded (TAB) semiconductor dice to a flexible circuit leadframe, which also serves as a printed circuit board. The semiconductor dice are tested as discrete units before being mounted, or they can be tested in the final circuit form after the TAB process. After testing and reworking, the flexible circuit leadframe is encapsulated. The dice and circuitry on the leadframe including the leadframe are encapsulated in a mold forming a single package body for the entire module. The disadvantage to this approach is that repair of the module after encapsulation is not possible. Hence, any failure of a semiconductor die inside the encapsulated module would cause the entire module to be rejected.